1. Field of the Invention
The invention relates to a method for production of diamond-like carbon film having semiconducting property, and in particular, to a technique for developing materials useful for a semiconducting diamond-like carbon film and the associated technique for designing its production process and the target raw material used. The invention also relates to a diamond-like carbon semiconductor and to its use in electronic and photoelectric devices and elements such as wires and electrodes in the solar cell, semiconducting elements, and the electronic components.
2. Description of the Prior Art
Diamond film and diamond-like carbon exhibit predominantly high visible and infrared (IR) transmission, high mechanical strength, high electric resistance, and resistance to corrosive gas or other medium, and consequently, they can be used as highly protective materials and anti-reflective coatings. Owing to the energy crisis, research in thin-film solar cells has attracted much attention. Among materials useful in solar cells, silicon crystals have unique semiconducting characteristics and therefore they can be used in semiconducting elements and solar cells. On the other hand, diamond materials although have atomic structure similar to that of silicon crystals, they belong inherently to an insulating material. Accordingly, a number of researchers have attempted to change the electrical property of diamond material into semiconducting or conducting nature by means of doping techniques so as to favor the application and development of diamond materials. Among those attempts, changing electric resistance of diamond film or diamond-like carbon (DLC) film by means of doping could make it possible for diamond film or diamond-like carbon film being applicable in semiconductor or electrical elements. Methods for lowering electric resistance of diamond film or diamond-like carbon included doping of hydrogen phosphide or diborane, blending to form metal film, nitrogen infiltration during film deposition and the like.
At present, most of the commonly used diamond films or diamond-like carbon films having semiconducting property are produced by processes or equipments based on chemical vapor deposition (CVD). However, in the production by CVD, it is indispensable to use expensive equipment and toxic or inflammable alkane gases to form and produce diamond film or diamond-like carbon film through high temperature reaction. In contrast to this, it is desirable to provide a method for the production of semiconducting diamond-like carbon film without using high temperatures, chemical reactions and toxic or inflammable gases.
Conventional techniques for producing diamond-like carbon thin film and diamond thin film having semiconducting properties can be summarized as following:                1. U.S. Pat. No. 7,393,717 disclosed fabrication of electric resistance and contact connection electric resistance regions, wherein the electric resistance region comprised conductive diamond. The electric resistance region was an electric resistance layer consisting of a small part of non-conductive diamond placed on the top of a substrate. The deposition of the diamond was carried out by CVD; microwave plasma enhanced chemical vapor deposition (MWPECVD), Hot Filament CVD or hydrogen/methane plasma. The diamond comprised a dopant therein in a manner that the diamond could become a p-type semiconductor. The dopant comprised at least one of the following components: boron, sulfur, phosphorus, diamond-like carbon (DLC), lithium, hydrogen, nitrogen or graphitic carbon (sp2-bonded carbon).        2. U.S. Pat. No. 7,223,442 disclosed a process for preparing diamond-like carbon nanocomposite thin film doped with basic elements such as carbon, silicon, metals, oxygen and hydrogen. The preparation process for the thin film could be described as follows: a substrate was placed in a vacuum chamber and the film was deposited under an external bias on the substrate of 0.3˜0.5 kV; as the gas plasma discharged, its carbon particles generated a plasma energy density of more than 5 kW-h/g-atom and were vaporized together in admixture with organic compounds into the plasma; the dopant particle beam was guided into the plasma; and a thin film was thus grown on said substrate to form a conductive nanocomposite thin film doped with carbon, wherein atomic concentrations of carbon, metal and silicon thereof were identical to a previous verified ratio. Said thin film was coated with a silicon dioxide layer. Passing a uni-directional electric current over the thin film could result in heat generation on the thin film. A nanocomposite thin film that had been doped with diamond-like carbon and had a multi-layer structure was thus prepared.        3. U.S. Pat. No. 5,635,258 disclosed a process for preparing boron-doped diamond thin film by microwave plasma chemical vapor deposition (MP-CVD). The binary reaction gas system used in the process had a composition of CxHy—CO2 and CxHyOz—CO2. The dopant used therein was trimethyl borate.        4. Taiwan Patent No. 594853 disclosed a method for producing diamond film by CVD. On the surface of the substrate, a dopant layer was formed by coating a solution of boron, di-boron tri-oxide, boron nitride and the like on the substrate, or by sputtering a solid state target material. Its last step comprised a heat treatment.        5. Taiwan Patent No. 591131 disclosed a method for producing diamond film by CVD. In this method, a gas mixture formed from B(OCH3)3 gas and raw material gases, using boron in the gases as the dopant, was deposited on a substrate through gas phase reaction to form diamond film; wherein the volume concentrations of B(OCH3)3 gas and the raw material gas were set to be in the range of 0 Vol. % to 8 Vol. %.        6. U.S. Pat. No. 6,939,794 disclosed a hard photomask consisting of boron-doped non-crystalline carbon, as well as a method for producing said photomask. The non-crystalline carbon photomask could provide improved corrosion resistance against various materials. Said boron-doped non-crystalline carbon thin film was prepared by PECVD. A semiconductor substrate such as a semiconductor wafer was used as a substrate, the chamber temperature was set in the range of 400° C. to 650° C., and propylene gas was introduced at a flow rate in the range of 300 sccm to 1500 sccm. At the same time, diborane gas was introduced at a flow rate in the range of 100 sccm to 2000 sccm. During the introduction of the propylene gas, the power was adjusted in the range of 100 W to 1000 W, and the pressure was set in the range of 0.4 Ton to 0.8 Ton.        7. U.S. Pat. No. 7,144,753 disclosed a nano-scale boron-doped diamond crystal having electric conductivity. The boron-doped diamond used boron within the crystal as the charge carrier. The diamond was used most effectively in the redox reaction of the electrochemical electrode, as well as in the elimination of aqueous solution. This patent revealed that doping of boron element can change the electrically insulating property of a diamond material into an electronic feature of an electric conductor.        
It is apparent that these prior attempts to incorporate hetero-elements into a diamond material to change the semiconducting characteristics of said diamond material to achieve a semiconducting property with high carrier concentration. Further, the preparation processes involved were mostly based on the synthesis of diamond material through chemical vapor deposition (CVD). The doping elements used therein come largely from the gaseous phase of trimethyl borate (B(OCH3)3) or methyl borate (C3H9BO3). In most cases, the doping gas was mixed with raw material gas and then the purpose of doping was achieved through a synthetic reaction process. Finally, diamond materials or diamond-like film was synthesized in manner using chemical vapor. It was apparent that conventional techniques employed expensive equipment for chemical vapor synthesis, and since they used mostly reactants in gaseous states, there were unfavorable factors such as parameter instability, danger involved in high temperature chemical reactions during synthesis and the like.
In summary, conventional and current techniques are not ideal and need to be improved urgently.
The inventor had learned the various disadvantages derived from conventional methods and devoted to improve and innovate, and finally, after studying intensively for many years, had developed successfully a method for producing diamond-like carbon film having semiconducting property and thus accomplished the invention. Accordingly, the semiconductor expected to be developed by the invention can be produced by a magnetron sputtering process under conditions without high temperature, chemical reactions and in the absence of inflammable or toxic gas. Further, said semiconductor can be a p-type semiconducting diamond-like carbon film (p-DLC). There has never been described a means to produce a p-type semiconducting diamond-like carbon film (p-DLC) by means of RF magnetron sputtering. Boron doping in this type of diamond-like carbon film production process can be demonstrated as the first-step of development in the semiconducting diamond-like carbon film and thereby is expected to enable the semiconducting electricity of the diamond-like carbon film to be better than that of diamond thin film. The Boron element is the most logical element to be doped in a diamond, since the atomic radius of boron is similar to that of carbon, which renders boron to substitute and infiltrate into the structure of diamond without significantly deforming its crystal lattice. In addition, it can be confirmed that the product is a p-type doped semiconducting diamond (p-Diamond), and thereby can greatly enhance the electrical properties of diamond thin film. Accordingly, the invention results in doping of boron in the coating of a diamond-like carbon film through radio frequency (RF) magnetron sputtering, whereby the surface area ratio of carbon target to boron tablet is used as a parameter to determine the influence of the doping level on the electric property of the semiconductor.